1. Field of the Invention
The present invention relates to a polygon scanner and a production process thereof.
2. Description of the Related Art
A polygon scanner is provided for beam deflection and scan in image forming apparatuses such as laser printers and digital copiers. It is required that the polygon scanner rotate at a high speed to meet a recent demand for higher image forming speed.
The polygon scanner of this type is for example constructed as follows. A plurality of recesses and protrusions for dynamic pressure generation, for example herringbone grooves, are formed on an outer peripheral surface of a fixed shaft. A cylindrical rotating shaft covers the fixed shaft with a certain bearing clearance to form a radial dynamic pressure air bearing as will be referred to simply as an air bearing. A polygon mirror or rotary multiple facet mirror is mounted on the rotating shaft. In this arrangement, either a dynamic pressure bearing or a magnetic bearing is employed as a thrust bearing for positioning and supporting the rotating shaft in the axial direction thereof. The fixed shaft and the rotating shaft both are made of stainless steel, and the pattern of recesses and protrusions for dynamic pressure generation is formed for example by etching. Since the polygon mirror is required to be finished at a high accuracy and to be light, aluminum is frequently used for forming the polygon mirror.
There is another type of polygon scanner using a bearing with an enhanced wear and abrasion resistance, which is not a non-contact bearing. In such a case, for example in order to make a difference of hardness between a rotating shaft and a fixed shaft, a suitable heat treatment is effected on either one of the rotating shaft and the fixed shaft, which has a lower hardness than the other so as to prevent seizing and to assure the wear and abrasion resistance. For example, Japanese Patent Laying Open Application No. 53-108461 describes a bearing device, which is not a non-contact bearing, in which a member supporting the rotating shaft is made of aluminum or an aluminum alloy, and in which an anodic oxide film is chemically formed on the member to improve the wear and abrasion resistance.
However, the former example of conventional polygon scanner, that is, the conventional dynamic pressure air bearing type .polygon scanner, has the following problems, because the rotating shaft is made of stainless steel.
(1) Since the rotating shaft (of stainless steel) has a high specific gravity, it is readily affected by vibration from disturbance, which causes the rotating shaft to contact with the fixed shaft made of a similar material so as to result in seizing. Thus, in order to prevent the seizing, the bearing clearance must be set at a high accuracy with an increased bearing stiffness, which requires a very high dimensional accuracy of each shaft, increasing the production cost.
(2) Since a thermal expansion coefficient (coefficient of linear expansion) of the polygon mirror is greatly different from that of the rotating shaft, heat during high speed rotation causes a slight deviation (of about several .mu.m) in a portion where the polygon mirror is mounted on the rotating shaft, which causes a problem of large vibration of the polygon scanner in high speed rotation. Thus, in order to prevent such a defect, the polygon mirror is inspected in assembly process while rotated at a high speed, which results in an increase of production cost.
(3) Since the rotating shaft has slight magnetism, fine dust is easy to attach to the rotating shaft. The fine dust could cause a damage on the bearing, so that a troublesome dust removing operation is necessary. This also increases the production cost.
The latter example of the conventional polygon scanner has the following problem.
(4) Since the polygon scanner is not of non-contact bearing type, it has a limit in speed increase, which cannot meet the recent demand for higher image forming speed.